1. Field of the Invention
The present invention relates to the in situ decontamination of soils using electric fields to transport strong oxidants, preferably peroxymonosulfate or peroxydisulfate, through the soil. In the practice of this invention, it has been found that the oxidant effects degradation of the contaminant in situ without effecting the mobilization of the contaminant and that the electric field ensures both transport and complete mixing of the oxidant and the contaminant as well as subsequent extraction of the resulting reaction products from the soil.
2. Description of the Prior Art
Peroxymonosulfate (HSO.sub.5.sup.-) and peroxydisulfate (S.sub.2 O.sub.8.sup.2-) are strong oxidants capable of degrading most organic compounds. Peroxymonosulfate is an effective oxidizing agent with a redox potential similar to hydrogen peroxide. It is more stable, easier to store and handle than H.sub.2 O.sub.2. Peroxydisulfate is an even stronger oxidizing agent than HSO.sub.5.sup.- and H.sub.2 O.sub.2. Both peroxysulfate compounds can be activated by contact with metal ions or by exposure to light to generate sulfate radical anions. The sulfate radical anions are sufficiently strong oxidizing agents to mineralize nearly all organic compounds (convert to carbon dioxide and mineral acids). Unlike hydrogen peroxide or peroxymonosulfate, peroxydisulfate can also generate sulfate radical anions upon the heating thereof. These peroxysulfate compounds are ideally suited for in situ soil remediation because they are water soluble and do not require light activation or the addition and mixing of multiple reagents. They are relatively inexpensive, and are easily handled, stored, and transported.
Peroxymonosulfate has heretofore been used to decontaminate aqueous waste streams containing contaminants such as cyanides, phenols, and lignin. The light catalyzed degradation of organic compounds with peroxydisulfate was mentioned in German Patent DE4330518A1, Blaschke et al, March 1995. Peroxydisulfate has been used in total organic carbon monitors due to its ability to convert virtually all organic matter to CO.sub.2 as detailed in G. R. Peyton, Mar. Chem., 1993, 41, 91. These disclosures demonstrate the ability of peroxysulfate compounds to destroy organic contaminants, but all of the oxidations are carried out in aqueous solutions. Accordingly, the use of peroxysulfate compounds in soil remediation applications is not intuitive since these oxidants would be expected to react nonspecifically with the soil and soil organic matter as well as the target contaminants. Furthermore, the use of peroxysulfate compounds would be expected to be limited to ex situ soil remediation due to limitations of mixing the aqueous treatment solutions in undisturbed soils contaminated with hydrophobic organic contaminants. In the case of peroxydisulfate oxidation of recalcitrant contaminants, the requirements of light-, metal-, or heat-activation to generate sulfate radical anions would seem to make in situ soil remediation all but completely impractical. Nonetheless, these limitations are overcome in the practice of the instant invention due to the unexpected specificity of the peroxysulfate compounds for oxidation of the organic contaminants and the ability of electric fields to generate heat and move the oxidants through low permeability soils.
A problem encountered when using aqueous solutions for the treatment of soils is a matrix effect in which the aqueous solution channels around the contaminated soil particles instead of flowing through them. Soils have an inhomogeneous distribution of microscopic and macroscopic pores. With time, hydrophobic organic molecules penetrate into the microscopic pore space of the soil and partition into hydrophobic soil organic matter (Luthy et al. Environ. Sci. Technol. 1994, 28(6), pp. 266A-276A; Readman et al. Sci. Total Environ. 1987, 66, pp. 73-94). Water added to the soil, either as a washing solution or a solvent for reactants, is repelled by these hydrophobic regions. Additionally, the wash solution takes the path of least resistance and percolates through the macroscopic pores and channels. The basic problem, then, is that the microscopic pores which contain residual amounts of the contaminants are inaccessible to the reagents added for remediation (Di Toro et al., Environ. Sci. Technol. 1982, 16, pp. 594-602; Carroll et al., Environ. Sci. Technol. 1994, 28, pp. 253-258). In order to remediate contaminated soils, a method of obtaining access to the microscopic pore space in the soil is required.
Electrokinetics is considered the only in situ remediation technology that is able to effect access to the microscopic pores of the soil (Trombly, Environ. Sci. Technol., 1994, 28(6), pp. 288A-291A.). In this technique, electrodes are inserted directly into the soil. The resultant electric field is evenly distributed within the soil and is insensitive to pore size (R. F. Probstein, R. E. Hicks, Science 1993, 260, 498.). The resultant electrokinetically-induced movement of ions and pore fluid through soil is relatively insensitive to pore size, so preferential channeling through the largest soil pores is avoided.
Electrokinetic phenomena within the soil are the result of three electric field-induced processes. The first process is electrophoresis, in which ionic colloids move toward the oppositely charged electrode. This process makes only minor contributions toward the total mass flux in consolidated soils. The second electric field induced process, electroosmosis, is of greater importance. The electroosmotic flow of pore fluid is the result of the electric field-induced movement of ions within the double layer formed at the interface of charged porous particles, such as clays, and ionic solutions. The movement of the electrified double layer draws bulk pore fluid along by viscous drag. In soils of high clay content (containing typically negatively charged clays) electroosmosis creates a net movement of pore fluid toward the cathodic electrode. The third of these three electrokinetic processes is electromigration. Electromigration involves the movement of soluble ions within the pore fluid toward the oppositely charged electrode. Generally, electromigration will be the dominant component of the total electric field-induced mass flux within the soil.
The relative contribution of electromigration and electroosmosis to the total mass flux in the soil is determined by the ratio of effective ionic mobilities to the electroosmotic permeability of the soil (Y. B. Acar and A. N. Alshabkeh, Environ. Sci. Technol., 1993, 27, 2638.). The effective ionic mobilities are constant for individual ionic species and are not dependent on soil pH or conductivity. The electroosmotic permeability, however, is dependent on the soil zeta potential and conductivity, and is therefore effected by pH changes and reactions occurring in the soil during electroremediation. The effective ionic mobilities of species in soils have typically been found experimentally to be 10 to 300 times greater than electroosmotic permeabilities. In other words, the contribution of electromigration to the total mass flux across electrified soils is substantially larger than the contribution from electroosmosis. In addition, electroosmosis creates an acid front that sweeps across the soil, and the relative contribution from electromigration would be expected to further increase in dominance as the soil becomes more acidic and conductive as the electroremediation process proceeds.
Conventional electrokinetic remediation technologies seek to extract contaminants from the soil using electroosmosis. The movement of pore water under the influence of the electric field removes soluble contaminants from microscopic pore space. Collopy, U.S. Pat. No. 2,831,804, April 1958, details the electroosmotic removal of salts from agricultural soils. Probstein et al., U.S. Pat. No. 5,074,986, December 1991; Acar et al., U.S. Pat. No. 5,137,608, August 1992; and Pool, U.S. Pat. No. 5,433,829, July 1995, detail methods for extracting metal contaminants from soils using electroosmosis. Kim et al., U.S. Pat. No. 5,098,538, March 1992, detail a method for removing metallic contaminants from soils using both electroosmosis and acoustic energy. The use of these technologies is limited to fairly soluble contaminants. Chang et al., U.S. Pat. No. 5,240,570, August 1993, detail the use of electroosmosis and surfactants to extract polychlorinated biphenyls from the soil. This technology can extract organic contaminants of low aqueous solubility, but it is still an extraction technique and the contaminants are not destroyed in situ. In contrast, the remediation scheme proposed in the instant invention utilizes electromigration to transport oxidants through the soil. Since electrokinetics is used to transport decontaminating solutions, not contaminants, the solubility of the contaminant is not a limitation. The contaminants are degraded in place and secondary treatments are not required. The primary purpose of the instant improved electrokinetic process is to overcome problems with low permeability and channeling which would otherwise prevent the decontaminating solution from reaching the contaminants.
Marks et al., U.S. Pat. No. 5,458,747, October 1995, detail the use of biological reagents in conjunction with electroosmosis to decontaminate soils. The electric field is employed in their technology to extract metallic and soluble organic contaminants, transport biological reagents, and transport nutrients and process chemicals. Their process is limited to the treatment of soils contaminated with readily biodegradable contaminants. On the other hand, the instant invention employs strong oxidants which have been shown to degrade virtually all organic compounds. Thus, there are fewer limitations on the contaminants that can be degraded.
In addition to problems with low soil permeability and contaminant insolubility, soil remediation is often hampered by the chemical inertness of the contaminants. Many soil contaminants are difficult to degrade chemically or biologically. Otherwise the contaminants would degrade due to natural attenuation. Recalcitrant contaminants, such as chlorinated organics, can only be oxidized by extremely reactive chemicals. Due to the inherent reactivity of these extremely reactive chemical oxidants, they have short lifetimes and must be prepared in situ. The oxidant of choice for utilization in the instant technology is the sulfate radical anion which can conveniently be generated in situ by applying moderate heat to a peroxydisulfate containing soil, Eq. 1, infra. ##STR1## Heat loss during electroremediation has generally been considered a waste of energy from the perspective of electrokinetic extraction of metallic impurities from the soil. Therefore, the prior art in electrokinetic soil remediation strove to use smaller currents and electric fields in order to make the process more energy and cost efficient. The present invention, in contrast, benefits from resistive heating within the soil. The heat generated by larger electric fields supplies the energy required to initiate the cleavage of peroxydisulfate into sulfate radical anions and desorb contaminants from hydrophobic pore space within the soil. Therefore, in remediating soils containing recalcitrant contaminants the present invention uses electric fields which are typically an order of magnitude larger than those utilized in the prior art.
A secondary objective which is realized as a result of practice of the instant, new, and novel electrokinetic process is to contain the reaction products and remove them from the soil in a controlled manner. Most electrokinetic remediation schemes taught and disclosed in the prior art are based on electroosmosis and only work well in packed clay soils. Increased contaminant leaching could occur if electroosmosis drives the contaminant from the clay into sandy areas or fractures in the soil (Acar et al., Waste Manage, 1993, 13, pp. 141-151). Since the method proposed in the instant new and novel process uses electromigration to deliver reagents rather than to mobilize the contaminants, only the peroxysulfate compounds are free in solution. The contaminants are still sorbed to the soil so the most undesirable risk of spreading the contamination is dramatically reduced. It is noteworthy that the peroxysulfate reagents are easily degraded and decompose to sulfate and water within days (Toennies, J. Am. Chem. Soc., 1937, 59, p. 555), since in instances when an area of the soil having a very high permeability is to be treated by practice of the instant invention and the electric field is unable to completely contain the mobile reagents during electrokinetic remediation, only relatively innocuous materials, such as sulfate, will break containment. In addition, electromigration across soils is at least an order of magnitude faster than is electroosmosis (Acar et al., Environ. Sci. Technol., 1993, 27, p. 2638; and Hicks et al., Environ. Sci. Technol., 1994, 28, pp. 2203-2210) and is efficient in both high and low porosity and in both saturated and unsaturated soils (Lindgren et al., Environmental Remediation, 1991). Therefore, leaching in arid, sandy, or heterogeneous soils is less likely. Furthermore, the continued migration and removal of expected reaction products, such as sulfate, from contaminated soils and aquifers using electromigration should be readily accomplished (Runnells et al., EPRI TR-104170 Project 8060, 1994).
As noted above, the instant invention offers several advantages over electroosmotic remediation processes. First, the process time, heretofore referred by such prior art processes, is dramatically reduced by the practice of the instant invention because the electromigration of ionic species across the soil occurs much more rapidly than transport by electroosmosis. Second, common problems that have plagued electroosmotic methods, such as the consolidation of the treated soil, the buildup of pH gradients, or the precipitation of the contaminant near the electrodes, are reduced because electroosmosis is only a minor contributing factor in the practice of the instant invention. Third, the increased efficiency of electromigration relative to electroosmosis allows a wider range of more reactive electrodes to be used since the electrodes will likely remain intact during the shorter duration of the electromigration process. This is an important consideration in instances where it would be convenient to use buried metallic objects, such as previously discarded 55 gallon drums or leaking underground storage tanks, as electrodes.